1. Field of the Invention
The present invention relates to a communication method, a transmission method, and a reception method suitable for application to a wireless telephone system according to cellular communication system, for example, and to a base station and a terminal based on these methods, and more particularly relates to those suitable for application of the TDMA (Time Division Multiplex Access) method.
2. Description of the Related Art
Various wireless telephone systems for wireless transmitting digital data signals are developed and put to practical use. A method for efficient connection which is practically used, is to apply the TDMA method. Said method allows a plurality of terminals to be connected with a base station in a single transmission band (frequency channel) by dividing the single transmission band into time slots on a time axis and allocating each time slot to the plurality of terminals (mobile station).
FIG. 1 shows an example of channel configuration for a wireless telephone system according to the cellular communication system using the conventional TDMA method. The ordinate indicates frequencies, and the abscissa indicates the lapse of time. The frequency axis has one band slot formed by every predetermined frequency band (for example, several kHz) and these band slots are numbered (herein, consecutively numbered). The time axis has one time slot formed for every predetermined period of time (for example, several hundreds xcexc sec) and these time slots are numbered (herein, consecutively numbered).
Predetermined band slots (in the example of FIG. 1, the thirteenth band slot and the twenty-third band slot) are allocated as band slots dedicated only to control channel CCH (Common Control Channel) transmission. In said band slots for control channel CCH, all the time slots are occupied with the control channel. In this connection, adjacent base stations have different band slots allocated to their control channels. For example, one base station performs its control channel CCH transmission at the thirteenth band slot, and another base station adjacent to said base station performs its control channel CCH transmission at the twenty-third band slot.
Other band slots (in the example of FIG. 1, the first to twelfth band slots, the fourteenth to twenty-second band slots, and the twenty-fourth to thirty-seventh band slots) are allocated as transmission bands to information transmission channel TCH (Traffic Channel) in which call audio data or other various data (hereinafter, referred to as user information) is to be transmitted. The setting of information transmission channel TCH at said transmission bands is communicated from a base station through the control channel to each terminal. Therefore, when being connected each terminal with the base station, a band slot in which the control channel CCH is transmitted, is received at first. The band slot and the time slot allocated to said terminal for transmitting the user information are determined based on the control information indicated by the control channel CCH transmitted in said band slot, so as to start the communication with the base station at said band slot and said time slot.
The thirty-sixth band slot in FIG. 1 shows an state of communication between the base station and a terminal. Referring to FIG. 2 which is an enlarged view of a portion of said band slot, said example shows three slots constituting one TDMA frame in which the communication is performed within three time slot periods. At a location of the time slot number 1, the transmission slot Tx for transmission of user information from the base station to the terminal has a length of one time slot period. At the next location of the time slot number 2, the reception slot Rx for reception of user information from the base station to the terminal has a length of one time slot period. At the next location of the time slot number 3, both the transmission and the reception pause for one time slot period. At the succeeding locations, the process of these three time slot periods (the process of one TDMA frame) is repeated. In this connection, the transmission slot and the reception slot are designated from the viewpoint of the base station, and of course, these slots are reversed from the viewpoint of the terminal.
Within a period over which the transmission and reception of user information pause (for example, the time slots number 3, 6, and 9), the search operation for available base stations is performed. That is, with a wireless telephone system according to the cellular communication system, areas where each base station can communicate are normally arranged in consecutive cells, and each terminal must determine which base station can communicate with it. Specifically, the control channel CCH transmitted from the base station is received by the terminal. Then, the terminal determines which base station it can communicate with based on the received field intensity or the like, and makes a request for connection with the base station with which the terminal can communicate. Therefore, each terminal must perform the search operation to search for a base station cell where said terminal can communicate at that point of time. Even after the communication starts, if the terminal moves into another area, the handoff process for switching the base stations for communication is required. Therefore, during a call, the search operation must be performed to find out available base stations from time to time.
The example shown in FIG. 2 shows the case where a frequency channel (band slot) is occupied with a terminal and however, depending on the setting state of period to arrange transmission slots Tx and reception slots Rx, a plurality of terminals can be connected with a channel. For example, another example of FIG. 3 shows eight time slots constituting one TDMA frame. The first of the eight time slots in said TDMA frame is a reception slot Rx for reception of user information from the terminal. The fourth time slot is a transmission slot Tx for transmission of user information from the base station. With this configuration, if the transmission and reception operations are performed at the same frequency, one TDMA frame allows four slots to be set for transmission and reception so that four terminals can share a band slot. Also in the example of FIG. 3, within a predetermined period of slot in each TDMA frame in which neither transmission nor reception of user information is performed by each terminal (herein, for example, the sixth and seventh time slots), the search operation is to be performed to find out available base station.
For the communication state shown in FIG. 2, if the incoming line and the outgoing line are at different frequency bands from each other, separately for each line, a terminal can communicate with the base station within a period of three time slots, and therefore three terminals can share a frequency channel (a band slot).
In either configuration, a period in which the cell search operation is performed for each TDMA frame is made available to each terminal. During said period, a process is performed to receive control channels. For such a configuration that the cell search operation is to be performed for each TDMA frame, it is not easy to deal with more frequent or less frequent cell search. In order to perform the cell search for each TDMA frame as shown in FIG. 2, one TDMA frame must be composed of at least three time slots and it is impossible to provide one TDMA frame composed of two time slots.
When communication is performed between the base station and the terminal in the configuration described above, appropriate control over the power of signals transmitted from the terminal can make the communication state better, as well as can be also desirable in power consumption of the terminal. That is, since a wireless telephone system includes a plurality of terminals which are located randomly, a terminal can make communication in the vicinity of any base station or a terminal can make communication at a location distant from any base station. Therefore, it is not desirable to perform all the transmission operations at a fixed level of power. If the transmission power from a terminal can be controlled appropriately, a lower transmission power is set when the communication is performed in the vicinity of the base station, and a higher transmission power is set when the communication is performed at a location distant from the base station. Thereby, waste of power consumption can be avoided during the transmission process at terminals, and the level of interference with other communication operations can be minimized. However, a complicated process is required to appropriately control the transmission power at a terminal, and the control of transmission power, in fact, is not performed generally.
In view of the foregoing, an object of this invention is to provide an efficient search for available base stations at a terminal.
Another object of the invention is to appropriately establish conmmunication between a terminal and a base station depending on the location of the terminal at the point of time.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.